Radioactive battery



1959 J. H. BIRDEN ET AL 2,913,510

RADIOACTIVE BATTERY Filed April 5, 1955 Mr r OHM LOAD INVENTOR.

John H. Birden and Kenneth 0. Jordan ATTORNEY nited tates atent OfiiceRADIOACTIVE BATTERY John H. Birden and Kenneth C. Jordan, Dayton, Ohio,

assignors to the United States of America as represented by the UnitedStates Atomic Energy Commission Application April 5, 1955, Serial No.499,543

2 Claims. (Cl. 136-4) The present invention relates to the generation ofelectrical energy from radioactivity, and more especially to a novelbattery or cell wherein the energy of the charged particles emitted by aradioactive isotope is converted into heat energy and then is convertedinto electricity, in amounts for doing useful work, by means of athermopile.

It has been heretofore proposed that very small electrical currentscould be generated by collection of the beta particles emitted from aradioactive isotope on a charged surface, that small currents may begenerated between two electrochemically dissimilar electrodes separatedby an ionizable gas by forcibly ionizing the gas with radiation andconnecting a load across the electrodes, and that electrons fromradioactive strontium be utilized to bombard a semi-conductor having alarge junction formed by an impurity therein. None of the proposeddevices have proved entirely satisfactory for practical power packs,principally becausethey are voltage sources of relatively high internalimpedances. Where large currents are required, the internal power lossdue to the in ternal impedance is so great that the batteries must bemade undesirably large in physical size to overcome their ineificientoperation. Moreover, relatively small currents have been obtained fromprior radioactive batteries, and they have been costly and diflieult toconstruct. Great care must be taken to reduce the radiation hazard tothe user or, associated equipment, and the amounts of radioactivematerial which can be used must accordingly be kept rather small.

With a knowledge of the difficulties associated with constructingradioactive batteries of the types known to the prior art, the inventorshave for a primary object of their invention production of a novelradioactive voltage source of inherently low internal impedance, so thatlarge amounts of useful power may be delivered by a source of reasonablephysical size. A further object is to provide a radioactive batterywhich creates no radiation hazard of any kind to the user or toequipment. Yet another object is to provide a source that is relativelysimple to construct with available structural materials. Other objectsand advantages of the present invention will become ap parent from thefollowing detailed description of certain preferred embodiments thereof,when read in connection with the appended drawings, wherein:

Figure 1 illustrates one embodiment of our novel electric cell;

Figure 2 shows an alternative, preferred form of the active portion ofour cell;

Figure 3 shows the outer envelope enclosing the portion shown in Figure2;

Figure 4 illustrates schematically the electrical circuit of our novelcell; and

Figure 5 shows schematically the thermal circuit of our cell.

According to our invention, electrical energy is generated by convertingthe energy of radioactive decay to 314 heat energy and then convertingthe heat energy to electrical energy. A radioactive material of highspecific activity is sealed inside a suitable container which contactsthermally the hot junctions of a thermopile-21 plurality ofseries-connected, alternately hot and cold thermojunctions. The coldjunctions of the thermopile are thermally insulated from the hotjunctions and from the inner container and are electrically connected inseries relationship with the hot junctions. Preferably, for developmentof maximum power, our cell is so designed that one-half the heatgenerated by radioactive decay of the source is transferred to the outercontainer by way of equal-resistance thermopile leads, and the internalseries resistance of the thermopile is made equal to the load resistanceto be connected thereto.

Referring now to Figure 1, in one form our cell may comprise a sphericalcapsule 1 containing an intensely radioactive material therein, athermopile of dissimilar metal leads 2, 3 provided with hot junctions 4and cold junctions 5, an outer container 6 enclosing the assembly, andthermal insulating material, not shown in the interest of clarity,interposed between capsule 1 and container 6. Each hot junction 4 isplaced in thermal contact with capsule 1, while each cold junction 5contacts container 6. The thermopile leads are brought out through tubesocket or base 8, which fits tightly into the bottom of container 6.Suitable materials of construction for this cell are shown in Table II,column 1.

Referring now to Figures 2 and 3, a preferred cell constructioncomprises a small cylindrical metal capsule 21 containing a source ofhighly radioactive material; two groups 22, 23 of thermocouples, eachcouple having a hot junction in thermal contact with capsule 21 and acold junction formed externally to one of the insulator plates 24, 25;an outer sleeve 31 surrounding the assembly for protection andinsulation from the outside air; end caps 32, 33 closing the ends of thesleeve; and mechanical spacers 28 to separate the closely-spaced wiresto prevent contact between wires. The thermal insulator, preferably alight powder filling the space between capsule and container, is notshown for clarity. The entire space between plates 24, 25 is normallyfilled with the insulator, which is not electrically conductive.External leads 26, 27 are provided from opposite terminals of thethermopile, the two groups 22, 23 of couples being electricallyconnected in series. Materials used in construction of the batteryillustrated are shown in Table II, column 2. The thermocouple wiresextend through parallel rows of holes in the base plates 24, 25, eachplate being formed of two half-discs held together by straps near theirperiphery. The end caps may preferably be hermetically sealed to sleeve31 and leads 26, 27 brought out through vacuumtight seals 34, 35. Eachcold junction may be cemented to the outer container by anelectrically-insulating, thermally conductive cement to provide a largesurface area for cooling of the cold junctions. The spacers 28 are notrequired if sufiicient tension is put on the wires to keep themseparate.

Referring now to Figure 4, the simple electrical circuit of thethermopile comprises a plurality of resistances, (r r represent theelectrical resistance per lead of each type of thermocouple wire in ohmsand r is the resistance per couple). The leads are connected in seriesforming 11 hot or cold junctions and the opposite ends being connectedto a load represented by n (1 +r ohms. We have found that maximum powerwill be developed from our radioactive battery when it is connected to aload resistance equal to the internal resistance of the battery. Thepower developed W will be given by the expression (ne) /4nr, where e isthe voltage generated per couple. The efliciency of the radioactivebattery may be found from the expression [Ef1=100 W /W =25 ne /W mlwhere W is the heat generated by the radioactivity withm the capsule,and r is the series resistance of both leads forming a couple. v I Wehave found that for maximum efiiciency, the leads of our thermopileshould have a thermoresistance equal to the sum of the thermoresistancesof the space between the capsule and the outer container and thethermoreslstance from the capsule to the hot junctions. We. have furtherfound that the diameter of each type of wire in the thermopile should beso chosen that the thermoresistance of each lead is the same, formaximum efliciency. 7 Referring now to Figure 5, the heat W generated byradioactivity within the capsule, travels from the capsule across thethermoresistance R from the capsule to the hot junctions, which are attemperature T From the hot junctions the heat travels along all thewires between 'hot and cold junctions, the thermoresistance of all thewires being represented by R to the cold junctions, which are attemperature T Some of the heat from the capsule also flows through thespace between the capsule and the outer container, the thermoresistanceof that space being represented by R ,All of the heat from the sourceexcept that converted to electrical power flows through the networkshown in Figure 5. L

The electrical resistivity pe and thermal resistivity p of the wires inthe thermopile 'are related by the Wiedemann-Franz constant If thediameter of each wire is chosen so that the thermal resistance r of eachlead is the same, the thermal re- V sistance R, of the n thermallyparallel leads is given by 7 2n For maximum efficiency, R should equalthe sum of R +R It may be seen that the maximum efliciency depends uponand varies inversely with Wiedemann-Franz constants of the thermopilewires selected. Therefore, the couple PI'OVIdlIlg the greatestthermopower is not necessarily the best choice for the radioactivebattery. If

R R +R then'it can be shown thatthe maximum efficiency 1 2) 1+ 2)( 1+ z)where B is the thermoelectric power of each thermocouple 1n volts/ C.and m is the ratio of R to R T and T are in degrees kelvin.

The efiiciency of the battery does not depend upon the number ofjunctions; that is, increasing the number of junctions does not providemore elficient utilization -of the heat. Moreover, the thermoresistancebetween the capsule and the hot junctions will increase slightly withthe number of junctions, thus actually decreasing the efficiency. Theinternal resistance of the battery will increase as the square of thenumber of junctions, while the total voltage developed will beproportional to the number of junctions employed.

In designing the radioactive battery for meeting specified requirements,the type of thermocouple to be used should be selected first. Sinceeificiency is proportional to the square of the thermoelectric power,selection of materials will be a most important factor. However, asabove stated, the Wiedemalnn-Franz constant and the resistivity of thethermocouple wires must be considered, as should be the melting pointsand the welding or soldering properties of the wires.Chromel-constantanand iron constantan have high thermoelectric poweran'd are entirely satisfactory in other respects. With a fixed number ofjunctions, and for maximum efficiency, the radius of the wire to be useddepends upon the Wiedemann- Franz constant and the resistivity, so thata Chromel wire, because of its much higher resistivity must besubstantially larger than the iron wire, Although the large mostpurposes the battery efiiciency with a Chromel junction will be higherthan with an iron junction.

The heat source should be next considered, important factors being theavailability of highly active materials, the specific activity of theisotope used, the relative ease of handling, including the dangerousradiations given oil, and the half life of the radioactive material. Anyradio active isotope could be used, but strontiumand polonium-210 appearto be most suitable from a cost per curie standpoint, and require aminimum of radiation shielding. Polonium-208 is ideally suited forbattery use except for its excessive cost. In construction of thesource, polonium may be volatilized intov a capsule or container whichis then closed with a plug and coated with nickel. Polonium has a-highspecific activity and gives off 5.4 mev. energy per disintegration. Thispermits the use of heat sources whose size and heat loss is determinedonly by mechanical considerations, and results in a. minimum number ofcuries of activity required for a given quantity of heat produced.Strontium-90 is available in sealed containers and is better suited fora longlived battery in that the useful life would be from 60 to 70'timesthe life of the P0 battery.

If a battery is to be designed to deliver a maximum power W into a loadR with a load current 1;, and a developed voltage of V the equations foroptimum battery design based on our battery having an efficiency of 0.2percent, are: i

ohms

I milliamperes V 9.4 10- when N is the number of junctions required, P0is the curies of polonium required, and Sr is the curies of strontiumrequired.

Table I lists two examples of our novel-batteries calculated from theseequations, based upon an efficiency of 0.2 percent attained by a batteryconstructed and tested.

TABLE I Po curies It is apparent that we 'have for the first timeprovided suitable and practical batteries for electric generationcapable of furnishing suflicient amounts of electrical power to becommercially useful, yet which are smaller, lighter, and can do morework than some dry cells. We have demonstrated that with relativelysmall physical dimensions we can provide an electric cell havingextremely long life, even while delivering full rated current, and whichcan produce sufficient amounts of current to operate, for example, atransistorized radio circuit such .as that shown in Radio and TelevisionNews, February 1953, page 37. Moreover, the output of our cells is notaffected by temperature of its environment so adversely as are drycells. 1

It will be apparent to those skilled in theart that the source ofradioactivity might be incorporated into the hot junction itself, ratherthan placed in a container and thermally connected to the junction.Itwill be further apparent that isotopes which emit only alpha particlesare to be preferred, since the problem of shielding the dangerousradiations therefrom is easily met, whereas sources which emit hardgamma and beta rays produce dangerous in... iii-0 radiation hazards. Formaximum efi'iciency, the source material shown should provide sufiicientenergy per unit volume, and should give maximum energy perdisintegration. Mechanical requirements presently set a minimum size forthe source capsule. The volume of such capsule is larger than the volumeof polonium-ZIO required, but substantially that required forstrontium-90.

Since the hot junctions must not make electrical contact with eachother, but must all be thermally connected to the source, heavyinsulating cement such as Sauereisen should be used for the hot and coldjunctions. The cement should provide at least fair heat conduction, goodTABLE II Battery No. l

Radioactivity- Capsule containing PO L 57 curies, P0 Sphere, O.D. 0.4 in

Battery No. 2

146 curies, Po Cylinder O.D. 0.21 in.,

length 0.45 in.

Material of capsule 0.047 in. cold-rolled steel with 0.02 in.cold-rolled steel with 0.02 in. nickel coating. 0.02 in. of nickelcoating.

Thermocouples Silver-soldered Chromel-con- Welded Chromel-constantan.

stantan.

Number of jimctinn 40.

Length of1eads 1.2 cm 1.3 cm.

Wire Siz S {B and S No. 18 Chromel. B and S No. 29 Chromel.

e B and S No. c0nstantan B and S No. 30 constantan. Insulation betweenjunctions and capsule Sauereisen cement Sauereisen cement. Estimatedm.-- 0.3.-.

Insulating material Santocel Santocel.

Outside containen- Lucite cylinder. Aluminum cylinder.

Internal resistance 0.25 ohm 15 ohms.

Voltage at no load.-- 42 millivolts 750 millivolts.

Te(%1pe{ a)ture rise from hot to cold junctions 42/7X7.7 10- =78 C750/X7.7X10- =244 C.

Temperature of cold junction (T2) C 80 0.

Temperature of capsule (based on m above) 146 C 373 Max. power delivered9.4 mllliwatts.

Activity of P0 in watts 4.65 watts.

Elficiencyuu 0.20 percent.

Wei ht 1 gins.

Work capacity 7.7 10f joules.

Current at max. power- 85 milliamps 2-5 Imlhamps.

electrical insulation, must be temperature-stable at very hightemperatures, and must provide good adhesion for structural purposes.

If hermetically sealed, and evacuated cells are not provided, a goodthermal insulator such as Santocel, a silica aerogel should be providedin the space between the source and the outer container.

Having thus described our invention, we claim:

1. A radioactive cell comprising an evacuated outer envelope, athermally-conducting capsule containing radioactive materialcharacterized by emission of radiation selected from the groupconsisting of alpha particles and beta rays disposed therewithin, and athermopile having two groups of alternate junctions and provided with apair of output terminals forming the cell output, one group of junctionsbeing in direct thermal contact with said capsule and electricallyinsulated therefrom and the other group being in thermal contact withsaid envelope whereby a temperature difference is maintained across saidthermopile.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Chemical and Engineering News, vol. 32, No. 42, Oct. 18,1954, pages 4183-4184.

RCA Atomic Battery.

1. A RADIOACTIVE CELL COMPRISING AN EVACUATED OUTER ENVELOPE, ATHERMALLY-CONDUCTING CAPSULE CONTAINING RADIOACTIVE MINERALCHARACTERIZED BY EMISSION OF RADIATION SELECTED FROM THE GROUPCONSISTING OF ALPHA PARTICLES AND BETA RAYS DISPOSED THEREWITHIN, AND ATHERMOPILE HAVING TWO GROUPS OF ALTERNATE JUNCTIONS AND PROVIDED WITH APAIR OF OUTPUT TERMINALS FORMING THE CELL OUTPUT, ONE GROUP OF JUNCTIONSBEING IN DIRECT THERMAL CONTACT WITH SAID CAPSULE AND ELECTRICALLYINSULATED THEREFROM AND THE OTHER GROUP BEING IN THERMAL CONTACT WITHSAID ENVELOPE WHEREBY A TEMPERATURE DIFFERENCE IS MAINTAINED ACROSS SAIDTHERMOPILE.